The present application claims priority under 35 U.S.C. xc2xa7119 to Japanese Patent Application No. 2001-16175, filed on Jan. 24, 2001. The contents of that application are incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to a drive force transmission apparatus disposed between a driving shaft and a driven shaft in a four-wheel-drive vehicle in order to transmit torque therebetween.
2. Description of the Related Art
There has been known a drive force transmission apparatus which has an annular frictional clutch provided between a front housing and a rear housing disposed to be coaxial with each other and rotatable relative to each other; and electromagnetic drive means which operates, upon being electrified, so as to bring the frictional clutch into a frictionally engaged state. The drive means includes an annular armature disposed on the inner side (front-end side) of the rear housing, and an annular electromagnet disposed on the outer side (rear-end side) of the rear housing. The electromagnet is fitted onto a tubular electromagnet support portion of a yoke to be accommodated within an annular recess formed in the outside wall of the rear housing. A predetermined inner gap is formed between the inner circumferential surface of the electromagnet support portion and the inner wall surface of the annular recess; and a predetermined outer gap is formed between the outer circumferential surface of the yoke and the outer wall surface of the annular recess.
Upon supply of electricity to the coil of the electromagnet, there is formed a closed magnetic path which passes through a portion of the yoke, the outer gap, an outer portion of the rear housing, a portion of the frictional clutch, the armature, a different portion of the friction clutch, an inner portion of the rear housing, the inner gap, and a different portion of the yoke. As a result, due to magnetic induction, the armature is moved toward the frictional clutch so as to press the frictional clutch, whereby the frictional clutch comes into a frictionally engaged state. By means of the thus-produced frictional engagement force, a main clutch mechanism is operated in order to couple the front housing and the rear housing in a torque transmissible manner.
The above-described conventional drive force transmission apparatus has the following drawback. In the drive force transmission apparatus, when electricity is supplied to the coil of the electromagnet, the rear housing and the yoke, both being formed of low-carbon steel (soft magnetic material of low carbon content), are magnetized to thereby serve as magnets. Therefore, if Fe-containing foreign matter (e.g., wear particles such as iron particles produced within a rear differential) enters the inner or outer gap, the foreign matter adheres to a surface of the rear housing and/or a surface of the yoke, which surfaces face the gap (hereinafter these surfaces will be referred to as xe2x80x9cfacing surfacesxe2x80x9d) and accumulate within the gap. In general, low-carbon steel is low in hardness, and therefore, when the front and rear housings rotate with foreign matter having entered the gap, the foreign matter wears or scratches the facing surfaces. As a result, wear particles are further generated and fill the gap, possibly resulting in the yoke and the rear housing becoming locked to each other due to seizure, thereby preventing rotation of a propeller shaft.
The present invention has been accomplished to solve the above-mentioned problems, and an object of the present invention is to provide a drive force transmission apparatus which has two members disposed with a gap formed therebetween to be rotatable relative to each other and which can mitigate wear and scratching of facing surfaces of the members, which wear and scratching would otherwise be caused by entrance of foreign matter into the gap.
The present invention provides a drive force transmission apparatus which includes a frictional clutch disposed between a first rotary member and a second rotary member disposed to be rotatable relative to each other; an electromagnetic drive mechanism for bringing the frictional clutch into a frictionally engaged state, upon reception of electricity, in order to couple the first and second rotary members in a torque transmissible manner; and a facing member disposed to be rotatable relative to the first rotary member with a gap formed therebetween, wherein a surface of the first rotary member and a surface of the facing member which face the gap each have a hardened surface layer.
Since a hardened surface layer is formed on the surfaces which face the gap, wear and scratching of the surfaces by foreign matter having entered the gap can be mitigated.
The drive mechanism may include an armature disposed within the first rotary member to face the frictional clutch and an electromagnet disposed outside the first rotary member to face the frictional clutch via a side wall of the first rotary member present between the frictional clutch and the electromagnet; and the facing member may support the electromagnet and may be disposed to form first and second gaps between the facing member and the side wall of the first rotary member at different positions with respect to the radial direction of the first rotary member.
Preferably, the hardened surface layer is formed by quenching, soft-nitriding, or plating.
The present invention provides another drive force transmission apparatus disposed within a carrier fixed to a vehicle body and adapted to drivingly couple first and second rotary shafts rotatably supported by the carrier, the drive force transmission apparatus which includes an outer casing coupled to the first rotary shaft, the outer casing being rotatable relative to the carrier; an inner shaft coupled to the second rotary shaft, the inner shaft being disposed coaxially with the outer casing and being rotatable relative to the carrier and the outer casing; a frictional clutch disposed in an annular space defined by the outer casing and the inner shaft, the frictional clutch entering a frictionally engaged state upon reception of a thrust force in order to couple the outer casing and the inner shaft together; and an electromagnetic drive mechanism disposed on one side of the frictional clutch with respect to the axial direction of the inner shaft, the electromagnetic drive mechanism including an electromagnet disposed outside the outer casing, a yoke supporting the electromagnet, the yoke being fixedly supported by the carrier in such a manner that the yoke faces a side wall of the outer casing with a gap formed therebetween, and a thrust force generation mechanism disposed inside the outer casing and adapted to produce the thrust force to be applied to the frictional clutch upon receipt of a magnetic force from the electromagnet via the gap and the side wall of the outer casing, wherein a surface of the yoke and a surface of the side wall of the outer casing which face the gap each have a hardened surface layer.
The electromagnet and the yoke may be received in an annular recess formed in the side wall of the outer casing, so that first and second gaps are formed between the yoke and the side wall of the outer casing at different radial positions.
The thrust force generation mechanism may include a cam mechanism disposed inside the outer casing to be located between the frictional clutch and the side wall, the cam mechanism including at least first and second cam members and generating the thrust force when the first cam member is rotated relative to the second cam member, the second cam member being fixed to the inner shaft; a second annular frictional clutch disposed inside the outer casing to be located adjacent to the side wall and adapted to drivingly couple the first cam member to the outer casing upon reception of a thrust force; and an annular armature disposed inside the outer casing in such a manner that the second frictional clutch is sandwiched between the armature and the side wall of the outer casing, the armature receiving a magnetic force from the electromagnet to apply a thrust force to the second frictional clutch to thereby bring the second frictional clutch in a frictionally engaged state.
Preferably, the hardened surface layer is formed by quenching, soft-nitriding, or plating.